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Yes, it is very possible.
As the Documentation says, you should have a minimum of three nodes in a Percona XtraDB Cluster. You only need one node out of the three nodes.
Make sure all nodes in the PXC have unique server_ids.
Make sure the Master you are replicating from has a unique server_id from the PXC nodes
Make sure the master and every PXC node is ...

PXC (and other Galera solutions) require one network hop at COMMIT time. If your application can combine writes into a single transaction, that will help at some level. However, still each COMMIT will take 300ms (or whatever it is).
Regular MySQL can do Master-Master and have both writable. That replication is asynchronous. However, there are a number ...

If I understand it correctly,
A transactional snapshot is taken of the chunk (~0 time)
The copy of that snapshot (old data) takes place (500ms)
Meanwhile, the trigger is hit by your UPDATE, and takes care of updating both the old and new tables. (This is where I get fuzzy.)
The bottom line is "trust it".

pt-online-schema-change takes advantage of the consistency nature of relational databases at table level. Let's see what happens when we write to the original table, one of three cases:
The chunk has already been copied. No problem, the trigger will overwrite the values of the accessory table within the transaction
The chunk has not yet been copied. The ...

Rather than trying to explain why most of the partitions seem empty, let me argue against that flavor of PARTITIONing.
To put it bluntly, PK & BY KEY(id, sha1) gains no performance, nor any other benefit that I can imagine.
Note that to get "partition pruning", you have to specify both the id and the sha_checksum. Performance will be essentially the ...

Create an EVENT to periodically UPDATE a stats table that stores the "last_ID" to which each of the 70 rankings was last pulled, and the current counts for each.
Each UPDATE on the stats table entries (one per ranking), should join the real table with the entries you are counting, and have the respective criteria, setting stats.count to ...